Distributor Rebuilding
You'll have to tear into the ones you have to see what's what. It's good that you have a selection to work with. If they have a lot of miles on them everything will be pretty trashed in terms of wear. The thing about bearings or in this case bushings, the wear is almost imperceptible for a really long time, and then it accelerates rapidly. Slowly at first, then all at once. Really have to use a dial indicator to check for runout.
On an ignition scope you can see this on the pattern, it's called "cam wobble", meaning the distributor cam that the point arm rides. This affects the dwell. I'd have to look it up in the book, but I'd be surprised if the allowable runout is more than .001" or .002". I messed around rebuilding a spare distributor and it turned out to be a much bigger pain in the you know what than it might seem at first, mainly because of parts availability, and excessive wear. This stuff is getting hard to find in serviceable condition.
One thing I discovered is simply replacing the oilite or bronze bushing alone may not be enough to bring it into spec, both the distributor shaft itself and bushing will likely need replacing. Then, this predictably or maybe not so predictably opens yet another issue because, the service replacement distributor shafts are not drilled for the oil pump drive gear, and this needs to be accurately located with a few thousandths of an inch. The only shaft type that appear to be available at least semi-routinely are the ones for the "comma" type centrifugal weights. The reluctors for this type of distributor are getting scarce. The problem with these, is the slots become elongated due to wear. These could be brazed up and filed back to correct dimensions.
The dimension of the distributor housing boss to the machined surface of the drive gear is critical, so mixing and matching gears & shafts & housing to make a serviceable unit is likely not possible, unless you get lucky. For some reason there is not much uniformity in the dimensions of the drive gear. In that sense each one is a custom build.
If you've a motor that won't quite idle smooth or has an annoying stutter at idle once in a while (among other tuning defects) it may just be a worn out distributor. This can sometimes be otherwise a bit hard to disgnose. Bushings eventually wear in an oval or egg shaped pattern, I was testing some home brew ignition condensers on a scope with a spare really worn distributor, and could clearly hear and see this ovoid track reproduced on the screen and coincide in the fluctuating idle like an out of balance washing machine spin cycle. This is one reason for the popularity of the Pertronix or other ignition modules, they are much less susceptible to this and will show an immediate improvement in idle quality.
After reading all this, you might begin to understand why I decided to camp out on eBay, and after several pots of coffee, a three day growth of beard I found a genuine pristine brand new NOS never-been-opened in the box MotorCraft Y-Block distributor for a hunnerd bucks and nearly broke the "Buy It Now" key.
